In a variety of situations, it may be desirable to determine whether metal objects exist that are obscured from view in some manner. The objects could include buried cables, such as power or communications cables, for example. Other objects could include buried unexploded ordnance and land mines. Buried unexploded ordnance and land mines pose a hazard not only to military personnel in time of war, but also to non-combatants during time of peace.
Electromagnetic induction has long been used to detect hidden or buried metal objects. Typically, a magnetic field generator propagates a primary electromagnetic field that in turn induces a secondary electromagnetic field in a nearby hidden object. A magnetic field sensor spaced from the hidden object senses the induced secondary electromagnetic field, and in the process, detects the hidden object.
A typical electromagnetic induction sensor 30 is illustrated in FIG. 1. The electromagnetic induction sensor includes a transmit coil 32 and a receive coil 34. A detection signal is provided to the transmit coil 32, which when transmitted, causes an electromagnetic field 36 to reach a nearby object 40. The electromagnetic field 36 induces eddy currents in the object 40, and a corresponding field 38 is emitted by the object. The corresponding field 38, also referred to as a detection signal, is received by the receive coil 34 along with an undesired coupling signal 50. The undesired coupling signal 50 results from the coupling of the detection signal transmitted by the transmit coil 32 into the receive coil 34.
The problem arises from the fact that the undesired coupling signal 50 may be thousands of times larger than the detection signal 38. For example, the detection signal 38 may be as low as −100 dB while the undesired coupling signal 50 may be as high as −10 dB. Since analog-to-digital converters have a limited voltage input and a limited resolution, the detection signal 38 is limited by how much it can be amplified. The detection signal 38 can typically be amplified a small amount due to the fact that the larger undesired coupling signal 50 will quickly reach the limits of the analog-to-digital converter. For example, if the analog-to-digital converter has a full voltage range of 0-5 volts and the undesired coupling signal 50 results in a 2 volt signal coupled into the receive coil 34, the total signal can be amplified 2.5 times, which leaves the detection signal 38 at a very low level.
One approach for removing the undesired coupling signal 50 is to use a quadrapole electromagnetic induction sensor 60, or what is more commonly referred to as a double-D coil design, as illustrated in FIG. 2. The quadrapole electromagnetic induction sensor 60 includes two receive coils 62 and 64. Each receive coil 62, 64 has a letter D shape. Since the field of the transmit coil 66 is coupled equally into each of the receive coils 62 and 64, the undesired coupling signal is nearly the same for each coil. An object to be detected under one of the receive coils 62 or 64 would not show equally in each coil. The receive coils 62 and 64 are summed together in a way that subtracts out much of the detection signal. However, this approach has several problems. There is a large dead spot in the center 68 between the two receive coils 62, 64 where no object can be detected. There are two separate areas of highest sensitivity, which results in making localizing or finding an object outline and location more difficult. Moreover, the physical gap 68 in the center between the two receive coils 62, 64 makes it nearly unusable in dual sensor systems where ground penetrating radar or other schemes are used to enhance detection or object discrimination.
Another approach for an electromagnetic induction sensor 70 uses a transformer 72 that is wound to match the coupling between the transmit coil 74 and the receive coil 76, as illustrated in FIG. 3. The detection signal is fed to one side 78 of the through the transformer 72 before being received by the transmit coil 74. The resulting signal is received on the other side 80 of the transformer 72, and is supposed to be nearly the same as the undesired coupling signal received on the receive coil 76. The resulting signal is then subtracted from the total signal as provided by the receive coil 76 on output line 82 to remove the undesired coupling signal. This method also has several problems. The transformer 72 needs to have a precise custom core and winding for each coil type. The transformer is rather large and heavy, as well as being costly. Moreover, the transformer is only typically matched for a particular coil in a particular environment, which nearly renders the transformer 72 ineffective when moving into different soil environments.